1. Field of the Invention
This invention relates to the field of fuel cells. More specifically, the invention comprises a novel reactant delivery system in which microjets direct a stream of gas against an impingement plate.
2. Description of the Related Art
Proton Exchange Membrane (“PEM”) fuel cells have undergone extensive development since the 1950's. The first practical application occurred during the Gemini space flights of the 1960's. While a complete explanation of the operation of PEM fuel cells is beyond the scope of this disclosure, the reader may benefit from a simple explanation.
FIG. 1 shows a prior art PEM fuel cell in schematic form. This particular fuel cell uses gaseous hydrogen and gaseous oxygen as its reactants. Proton exchange membrane (“PEM”) 14 lies at the center of the device and is the key to its operation. When properly conditioned, this membrane will allow hydrogen ions to pass through, but will not allow the passage of electrons. In operation, hydrogen inlet 18 supplies hydrogen gas (a “reactant”) to hydrogen manifold 30. Catalysts such as platinum or palladium strip the electrons from the hydrogen atoms to form hydrogen ions and free electrons. The catalysts are typically located on the exterior of the PEM.
The hydrogen ions flow through porous anode 12, through PEM 14, and into porous cathode 16. At that point the hydrogen ions combine with oxygen supplied by oxygen manifold 32 to produce water. Oxygen inlet 20 supplies a suitable flow of gaseous oxygen.
The free electrons are unable to pass through PEM 14 because the membrane is electrically insulating. They are forced instead to flow through an electrical circuit including electrical load 22. Electron flow 24 therefore provides electrical power to an external load, which is the primary purpose of the fuel cell. The “waste product” is water, which obviously poses no environmental concerns.
FIG. 2 shows a section through anode 12, proton exchange membrane 14, and cathode 16. These three components are typically laminated together to form membrane electrode assembly (“MEA”) 26. The anode and cathode are typically very thin (less than a millimeter). They may actually be formed by vapor deposition or electro deposition processes. Because the entire MEA must allow the passage of hydrogen ions, the anode and cathode (collectively referred to as the “electrodes”) must be porous.
The catalyst or catalysts are often also formed on the exterior of the MEA itself. Gas diffusion layer (GDL) is also typically added to the MEA's exterior in order to evenly distribute the fuel and oxidizer over the catalyst. The inclusion of the GDL can reduce the amount of catalyst needed.
The reader having an interest in further details regarding the nature of MEA's is referred to U.S. Pat. No. 3,134,697 to Niedrach (1964) and U.S. Pat. No. 6,099,984 to Rock (2000). Both these patents are hereby incorporated by reference in this disclosure.
FIGS. 1 and 2 provide a basic explanation of PEM fuel cell operation. However, as one might reasonably expect, the physical realization of the device is much more complex. Because fuel cells were critical to long term operations in space, fuel cell development was a critical obstacle in the moon race of the 1960's. PEM and MEA development took many thousands of man-hours.
FIG. 3 shows a simplified depiction of a PEM fuel cell. Membrane exchange assembly 26 includes a proton exchange membrane 14 sandwiched between anode 12 and cathode 16. The anodes and cathodes are depicted as hatched lines to indicate their thin and porous nature. The depiction is not intended to show what the anode and cathode actually look like. MEA 26 would naturally include other components as well. Current collection grids and conduits would be attached to the anode and cathode. Sealing gaskets are also used. For purposes of visual clarity, these components have not been illustrated.
Membrane exchange assembly 26 is sandwiched between hydrogen manifold 30 and oxygen manifold 32. Hydrogen inlet 18 supplies hydrogen gas to the hydrogen manifold while oxygen inlet 20 supplies oxygen gas to the oxygen manifold. Hydrogen and oxygen are considered the “reactants” for this type of fuel cell.
Mechanical features are often included in the prior art to facilitate clamping the assembly together. A series of mounting holes 42 pass in alignment through all the components. Bolts can be passed through these holes and nuts will then be tightened to clamp the assembly firmly together.
Of course, if one merely feeds gaseous reactants into the manifold, the fuel cell will not operate. The reactants must be ionized, and this is typically done by a catalyst placed on the anode and cathode. Palladium is a typical catalyst which can be deposited as a thin layer on the anode and cathode.
Serpentine passage 34 is cut into the face of the oxygen manifold which bears against MEA 26. The serpentine passage allows the oxygen to flow smoothly over the catalyst, which may be deposited on the surface of the serpentine passage, the cathode surface, or both. The serpentine passage is needed to hold the gas and the catalyst in contact for a time sufficient to allow ionization.
A similar serpentine passage is cut into the surface of hydrogen manifold 30 which faces the MEA. The serpentine passages have been used in fuel cells for many years. However, their efficiency is limited. The catalyst is often exhausted in the proximity of the gas inlet long before it is exhausted in the “tail” of the serpentine passage.
In state-of-the-art fuel cells, the hydrogen and oxygen manifolds are often made of graphite. The manifolds themselves may therefore be used as electrodes, eliminating the need for separate components. The manifolds are placed in direct contact with opposing sides of the MEA so that they can conduct the electrical current created by the reaction within the fuel cell. The mounting holes may then be used to house electrical conductors, with external clamping means being used to assemble the fuel cell.
Those skilled in the art will know that the operation of a prior art fuel cell is limited by several factors. First, the appropriate amount of water must be maintained in the membrane electrode assembly in order to keep the MEA “soaked” (critical to its operation) yet not “flooded” (which will destroy the membrane's operation). This is true for most existing MEA's, with Nafion being the most commonly used PEM material.
Second, power output is often limited by internal heat generation. The fuel cell generates internal heat across the MEA and this must be dissipated. Too much heat will damage the fuel cell. Thus, increasing power is not simply a matter of pumping in more reactants. The reactant flow must be limited in order to limit the generation of heat. Thus, a fuel cell construction which reduces or absorbs heat generation would be highly beneficial.